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Novel Nonreactive Diluent Reduces Cost and VOCs

In recent years, the growth of VOC-compliant coatings has increased dramatically due to requirements designed to reduce the amount of ozone formed in the lower atmosphere through solvent emissions. One of the predominant approaches in achieving VOC compliance has been through the use of high-solids polymers. As a result, there are many types of high-solids polymer technologies designed to assist formulators in producing low-VOC coating formulations to meet these impending regulations.

However, all formulators must now cope with the primary question of how to reformulate their conventional solids technologies, or perhaps to lower the VOC of an existing high-solids product, without imparting undesirable compromises in coating performance. In addition to higher cost, the high-solids alternatives to conventional solids polymers often exhibit performance and/or application problems due to necessary changes in polymer composition imparted to achieve lower solvent content. Typically such changes result in lower molecular weight, lower glass-transition temperatures, and a higher degree of functionality. Thus, the task of reformulation begins.

In many cases, there is now an alternative to complete coating reformulation with the introduction of the first in a series of new low-viscosity, low-color, chemically inert, substituted aromatic diluents. Sure Sol-300 is a chemically inert extender that could impart significant reduction in VOCs and substantial cost savings without any notable loss of performance when used as a modifier for various thermoset coating systems.

The diluent is a water-white liquid with a molecular weight of about 250. Its low viscosity allows for rapid viscosity reduction, while its high degree of aromaticity imparts additional resistance to corrosion and water-related exposures. Perhaps the most unique feature of this diluent is its low initial color and color retention. This now allows for the use of a chemically inert diluent in gloss enamel formulations based on a variety of coating technologies, including two-component (2K) epoxy coatings and 2K polyester or acrylic urethanes.

It should be noted that the concept of using nonreactive diluents is not a new approach to reducing the viscosity of polymer blends. One of the primary means of reformulation used in achieving VOC-compliant coatings in recent years has been to modify the existing polymer system with a reactive or nonreactive diluent to replace polymer and/or solvent. This has been particularly true in the reformulation of thermoset coatings, such as 2K epoxies and polyurethanes.

For years, a variety of relatively inexpensive, chemically inert, low-molecular-weight polymers have been used to lower the viscosity and extend the potlife of various types of solvent-based and 100%-nonvolatile epoxy products. Most of the more commonly used materials are hydrocarbons, such as liquid coumarone indene resins, aromatic hydrocarbon oils and resins, and benzyl alcohol. With the exception of the benzyl alcohol, most all other product types tend to be somewhat dark in color, thus limiting their use to those applications where aesthetic value was of little concern.

The use of chemically inert diluents in other types of thermoset coatings, such as 2K polyurethanes, has been greatly limited due to their adverse effect on crosslink density of the cured film and color retention, which are among the most important characteristics of such coatings. Thus, reactive diluents are more commonly used despite the high cost and unavoidable compromises in coating performance that commonly result.

In general, all of the various chemically inert, or nonreactive, diluents commonly used impart similar property improvements and degradations when used to modify a chemically reactive/thermoset coating system. Following are the typical results of incorporating a liquid, chemically inert diluent.

Typical usage levels for such products are usually limited to 10% to 15% by weight of the coating vehicle due to the aforementioned limitations and, often times, the desired VOC reduction is not achievable due to the relatively high viscosity of the diluent in question.

Coating Formulation

Following is an overview of studies conducted with Sure Sol-300, with focus on the benefits that can be achieved through use of this material in the formulation of high-performance, VOC-compliant, thermoset coatings. The studies also demonstrate how this diluent technology can allow the formulator to minimize compromise in performance while achieving substantial VOC reduction and cost savings. Our studies will show that the key to successful use of chemically inert diluents is to focus on achieving the necessary performance strengths for the intended market of the product while minimizing compromises in overall coating performance.

Prior to initiating a full evaluation of this material, a study was conducted to determine the degree of diluent loss when tested for volatile content as per ASTM D-2369 (EPA Method 24). Sure Sol-300 has a relatively high boiling point, which would presumably result in high retention of this material in the cured coating film when cured at ambient conditions. As allowed per ASTM D-2369 for 2K systems, the two coating components were blended, weighed into an aluminum dish, covered with PM acetate, and were allowed to stand at room temperature for 24 hours prior to actual volatile testing. Results for diluent modified epoxy and polyester coatings are shown in Table 1. As the data indicates, the Sure Sol-300 should not be regarded as a volatile in these coating types despite its expected volatility, as all samples tested revealed no loss of diluent when tested as per ASTM D-2369. All indications are that this material remains a permanent part of the cured film when these coatings are used under typical exposure conditions.

Once this was established, a comprehensive study was undertaken to determine the efficiency of this proprietary substituted aromatic modifier as a diluent in industrial coatings. Two widely used thermoset coating types were selected for study. The coating technologies selected were a high-solids, 2K epoxy primer system and a 2K, gloss-white enamel based on a conventional solids polyester/aliphatic isocyanate combination.

The high-solids epoxy technology was selected for two basic reasons. Of primary importance was the fact that these coatings are typically characterized by a high crosslink density, which results in extremely hard, solvent-resistant films that typically lack a high degree of flexibility. Secondly, the VOC of coatings in this class, while low, would have more long-term marketability if the VOC could be further reduced.

The high-solids, epoxy resin system chosen for this study was a resin/curing agent combination that is known to produce high-performance, VOC-compliant coatings for a variety of metallic applications. The epoxy resin component is based on a blend of a conventional 1001-type solid epoxy resin and basic bisphenol-A liquid epoxy resin at a ratio of 75:25 on weight solids. The resulting blend has an epoxide equivalent weight (EEW) of 425. The solid epoxy resin selected was EPON 1001 H75, which is 75% NV solution supplied in glycol ether PM. The advantage of such a blend relative to the unmodified liquid epoxy used in many other epoxy formulations is that the 1001-type epoxy resin, being a hard resin film former, provides dry times more comparable to those produced by conventional solids, high-VOC systems. The curative used was Ancamide 801, from Air Products and Chemicals Inc. This hardener is a relatively low-viscosity, 100% NV modified polyamide with an amine hydrogen equivalent weight (AHEW) of 145. This curative was selected due to its overall performance and the fact it has consistently been shown to be a 100%-nonvolatile material, as tested per ASTM D-2369 VOC test.

Standards for comparison for this phase of study were an unmodified epoxy primer control and a primer with a 10% modification using a hydroxyl functional hydrocarbon resin said to impart excellent viscosity reduction, longer potlife, and improved adhesion.

In selection of the polyester/aliphatic isocyanate combination, it was determined that studies should be conducted using conventional solids materials in an effort to substantially reduce the VOC and the formulated cost of these types of coatings.

The polyester/aliphatic isocyanate combination chosen was a conventional solids technology known to produce high-performance coatings for a variety of applications. Coatings based on this system are characterized by their long-term light stability, gloss retention, and chemical and abrasion resistance. These coatings are recommended for application to substrates including metal, concrete, wood, plastics and paper.

The primary polyester resin used was the Desmophen 651A-65, supplied by Bayer. This polyester is supplied at 65% NV in propylene glycol monomethyl ether acetate (PM acetate). Formulations also contain a very low level of Desmophen R-221-75, which is a 75% NV polyester employed to assist in pigment dispersion. The aliphatic polyisocyanate prepolymer used was Desmodur N-75, an HDI-based crosslinker supplied at 75% NV in a 50/50 blend of n-butyl acetate and xylene. This isocyanate is among the most commonly used biurets commercially available for coating applications. For this portion of study, the unmodified control formulation was prepared as a standard for comparison.

In each case, the base coating formulations used for these studies were those supplied by the polymer manufacturers. These formulations served as a baseline for diluent modification and as standards for comparison. Modification levels selected for the epoxy were 10% and 20% of total vehicle solids. The levels for the polyester/isocyanate system were 5%, 10% and 20% of total vehicle solids. As a result, the stoichiometry of each technology remains unchanged, and all prepared coatings will have the same total weight solids and comparable volume solids prior to reduction to application viscosity (see Table 1).

Coating Application

All epoxy primer compositions were blended and allowed 15 minutes induction prior to reduction to an application viscosity of 65 Kreb units. Coatings were then applied by conventional air spray to iron-phosphate-treated cold-rolled steel and grit-blasted hot-rolled steel to a dry film thicknesses of 2.0 to 2.5 mils and 3.0 to 3.5 mils, respectively. All coated panels were allowed to cure at room temperature for one week prior to any testing, other than early hardness and gloss measurements.

The polyester/isocyanate compositions were blended and reduced to a viscosity of about 60 Kreb units prior to application. These coatings were applied by conventional air spray to iron-phosphate-treated cold-rolled steel to a dry-film thicknesses of 2.0 to 2.5 mils. Each of the coated panels was allowed to cure at room temperature for one week prior to any testing, other than early hardness and gloss measurements.

Test Results

In review of the coating performance data, the contributions of diluent modification in the 2K, high-solids epoxy primer will be initially addressed. An overview of the resulting test data is exhibited in Tables 2 through 4. This data indicates that the use of the new substituted aromatic diluent compares favorably with that of the standard, unmodified epoxy primer, and outperforms the hydroxy-functional hydrocarbon resin. In fact, the data indicates that both the 10% and 20% substituted aromatic diluent modification levels are viable options in lowering the coating VOC without notable significant compromise in coating performance.

The key is in producing a coatings formulation that exhibits the necessary performance criteria for a given market. In review of the basic performance criteria for a general purpose, high-solids epoxy primer for structural steel applications, the following would typically be considered the most crucial aspects of performance.

It should be noted that while high crosslink density is characteristic of most high-solids epoxy/curative combinations, the products of this reaction are not key to the successful performance of a high-solids epoxy primer. In other words, the high degree of abrasion, solvent, and fluid resistance are not as crucial aspects of performance in a primer as they would be in a high-solids enamel intended for concrete flooring application.

In most industrial maintenance primer applications, solvent and fluid resistance are not key factors in coating selection due to the minimal risk of prolonged exposure to these materials. The primer must only exhibit the dry and cure required to allow for acceptable recoating with a solvent-based topcoat without lifting or softening to the extent that the topcoat fails to cure or have the expected surface aesthetics. Thus, in the formulation of a VOC-compliant, high-solids epoxy primer, all levels of this new substituted aromatic diluent exhibit acceptable performance for the intended applications.

In fact, with the exception of solvent and fluid resistance, the 20% level of the substituted aromatic diluent actually demonstrates the best balance of performance for a high-solids epoxy primer. Advantages exhibited by the 20% modification with the substituted aromatic diluent include a significant reduction in VOC at application viscosity, improved de-ionized water immersion resistance, a 7.5% reduction in vehicle raw material cost (RMC), and little or no compromise in any facet of coating performance crucial to the design of a high-performance epoxy primer intended for structural applications.

In review of the performance of the modified polyester/isocyanate coatings, it is imperative to note that the primary concern going into this phase of study was that the coating would lose an unacceptable degree of crosslink density and/or an unacceptable degree of discoloration with UV exposure, due to the strong aromatic character of this diluent. However, as indicated in Tables 5 through 7, it is apparent that the compromise in performance required to achieve lower coating VOC at a lower cost was quite moderate with low levels of the substituted aromatic diluent.

In most cases, the 5% and 10% Sure Sol-300 levels impart acceptable compromises for nearly all conventional application areas. In nearly all other cases, compromises in performance are insignificant in light of the VOC reduction achieved. Most notable among the numerous performance strengths of these coatings was the retention of film aesthetics, hardness, flexibility, solvent resistance, and abrasion resistance while imparting significant reductions in VOC and vehicle RMC. In fact, the 10% modification level reduces total vehicle cost by 6%, which equates to a decrease in vehicle RMC of about $0.94/gallon, while maintaining a performance profile nearly identical to the unmodified standard. In addition, VOC was reduced by 0.3 pounds per gallon. While VOC is only reduced by about 0.17 pounds per gallon with the 5% diluent modification, the cost savings available through use of this chemically inert diluent makes it a viable alternative in the formulation of such coatings if lower VOC is not of primary concern.

Summary

This new diluent could be an alternative to total coating reformulation to meet VOC requirements. It has been shown to have many features uncharacteristic of conventional chemically inert diluents, in that it is both very low in viscosity and very efficient in reducing solvent content, and also imparts the good color retention and UV stability to be used in highly weatherable coatings. In addition, volatile testing indicates that despite the low molecular weight of this diluent, it remains a permanent portion of the cured film when used in ambient-cure, 2K thermoset coating systems.

It has been shown that the key to successful use of a chemically inert diluent is to focus on the key performance criteria for a given market, while minimizing compromise in overall coating performance. Studies conducted have shown that the requirements of a high-solids epoxy primer can easily be retained even when using as much as 20% of this diluent based on total vehicle solids. More surprising was the overall performance of this diluent in 2K polyester/isocyanate coatings wherein up to 10% of total vehicle solids can be replaced without significant compromise in any facet of coating performance. The resulting coating demonstrated comparable hardness, color and UV resistance, and solvent and abrasion resistance, which are all key features of urethane coatings relative to an unmodified standard. In addition, the 10% level also imparts a decrease in VOC of 0.3 pounds per gallon and a reduction in vehicle cost of $0.94 per gallon in the formulations tested.

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